1. Field of the Invention
The present invention relates to a standard zoom lens and, more particularly, to a compact standard zoom lens with a high zoom ratio.
2. Related Background Art
In recent years, in exchangeable lenses for 35-mm still cameras, it has become a general practice to provide a zoom lens in place of a standard lens (a lens having a focal length of about 50 mm in the case of 35-mm format cameras). The zoom lens usually has a zoom ratio of about x2.5, a standard field angle, and includes two lens units having negative and positive refractive powers, respectively, in order from the object side (hereinafter referred to as a standard zoom lens).
Therefore, since the above-mentioned standard zoom lens is carried while being attached to a camera body as a lens to be ordinarily used, not only must it have a minimum required compact structure as an indispensable condition, but also it must be inexpensive while maintaining sufficient imaging performance.
In order to realize such a so-called standard zoom lens, the above-mentioned negative/positive two-unit zoom lens is the most suitable lens type, and various proposals have been made.
For example, Japanese Laid-Open Patent Application No. 52-135750 proposes a two-unit zoom lens which has a first lens unit constituted by three lenses, i.e., negative, negative, and positive lenses. On the other hand, Japanese Laid-Open Patent Application No. 4-251211 proposes a two-unit zoom lens which decreases the number of constituting lenses to further reduce the lens size and cost, and realizes a high zoom ratio.
Furthermore, Japanese Laid-Open Patent Application No. 5-249376 proposes a two-unit zoom lens which adopts a first lens unit constituted by two lenses, i.e., negative and positive lenses, so as to further reduce the lens size and cost.
However, the zoom lens disclosed in Japanese Laid-Open Patent Application No. 52-135750 is large in size and requires a large filter size, and has a zoom ratio as low as 1.8 to 1.9. Therefore, this zoom lens is inferior to modern zoom lenses in terms of portability and specifications.
The zoom lens disclosed in Japanese Laid-Open Patent Application No. 4-251211 has an improved design to attain a certain decrease in size and a decrease in the number of constituting lenses, and the zoom ratio can cover a range from about x2.5 to about x2.7. However, in this zoom lens, the attained size reduction is not sufficient, and a further size reduction is desired. In particular, this zoom lens has a refractive power arrangement, so that the total lens length at the telephoto end becomes shorter than that at the wide-angle end, and a further size reduction at the wide-angle end and a decrease in diameter are not still attained.
Also, problems associated with aberration correction such as a large variation and absolute amount of the spherical aberration upon zooming, insufficient correction of the curvature of field, and the like remain unsolved. Therefore, a high-performance zoom lens whose size and diameter can be further reduced without considerably increasing the number of constituting lenses, and can solve the above-mentioned problems associated with aberration correction, is demanded.
Furthermore, Japanese Laid-Open Patent Application No. 5-249376 discloses a negative/positive two-unit zoom lens which has a first lens unit constituted by a smaller number of lenses, as described above. The zoom lens disclosed in Japanese Laid-Open Patent Application No. 5-249376 realizes a high zoom ratio and high performance while further reducing the size, in the optical axis direction, of the first lens unit. However, since the first lens unit is constituted by two lenses, the refractive power of the negative lens in the first lens unit becomes relatively strong, and the deviation (the difference, in the optical axis direction, between the mother spherical surface in a paraxial region and the aspherical surface) tends to be relatively large.
Therefore, it is difficult to manufacture the above-mentioned negative lens using the current manufacturing methods such as a method of manufacturing the negative lens as a compound aspherical lens consisting of glass and a plastic material, a method of manufacturing the negative lens by glass direct press, and the like. For this reason, not only the manufacturing cost increases, but also it tends to become difficult to ensure certain tolerances as to deterioration of performance caused by water/moisture absorption, a change in temperature, and the like of a resin portion in association with aberration requirements in the compound aspherical lens even if such a lens can, in fact, be manufactured.